When using glass fiber to reinforce cement there is a loss of flexural strength and ductility as the cement cures, (ages), or as they say hydrates. This occurs in a moist environment which includes almost everywhere except in a conditioned space where the humidity is controlled, or the desert.
What causes this is as the cement cures it develops a number of by-products one of which is calcium hydroxide. This material migrates into the bundle of filaments, (strand), and physically glues all of them together reducing their ability to accept a bending strain. The strand then loses some reinforcing efficiency, and since the strands are oriented in all directions the resulting loss is significant. For example, the flexural strength diminishes from about 3600 psi. to 1200 psi., and the strain capacity down to 0.04 from 1.0.
Numerous solutions have been tried by the glass companies, cement companies, scientists and others. All solutions have tried to interrupt the movement of calcium hydroxide into the strand. Special coatings have been applied to the glass strand, pozzolans have been added to the mix, new cements have been developed and polymers have been added to the mix. All have some negatives associated with them so that they are not universally accepted, or satisfactory in production. For example, new cements have minimized the development of calcium hydroxide. But, this is not popular because the new cement has a very strict curing requirement and is expensive. Another solution adds metakaoline and a polymer to portland cements. This yields somewhat improved retained strengths.